Ultrasonic sensors for monitoring the condition of a vascular graft

ABSTRACT

A parameter indicative of the status of fluid flow is remotely monitored in a vessel, a natural graft, or a synthetic graft. One or more transducers are provided either in a wall of a synthetic graft or adjacent to a vessel or natural graft to monitor the parameter. A conformal array transducer or a tilted element is used to monitor fluid flow or velocity through the graft or vessel based on the effect of the fluid on ultrasonic waves produced by the transducers. The conformal array transducer comprises a plurality of elements that curve around the graft or vessel and are excited with an input signal provided by an implantable electronics circuit, producing ultrasonic waves that propagate into the fluid flowing within the graft or vessel. Transit time or Doppler measurements are made using an appropriate number of these transducer to determine either fluid flow or velocity. Various implantable electronic circuits are provided that enable a selected transducer to be driven and to receive an ultrasonic signal or a pressure signal indicative of the status of fluid flow monitored by the transducer. The implanted electronic circuit is connected to an implanted radio frequency (RF) coil. An external coil that is connected to a power supply and monitoring console is coupled to the implanted RF coil to convey power and receive data signals from the transducer that are indicative of the parameter being monitored.

This application is a divisional application, based on prior copendingapplication Ser. No. 08/949,413, filed on Oct. 14, 1997, and now U.S.Pat. No. 5,807,258, the benefit of the filing date of which is herebyclaimed under 35 U.S.C. § 120 and 37 C.F.R. § 1.53.

FIELD OF THE INVENTION

The present invention generally relates to the use of ultrasonictransducers to monitor flow and velocity, and more specifically, to theuse of such sensors to monitor flow and velocity of blood through agraft, so as to determine the condition of the graft.

BACKGROUND OF THE INVENTION

A section of the vascular system in a patient's body that is diseased ordefective can be surgically excised and replaced with a graft. A graftmay comprise a portion of another vessel extracted from a differentlocation in the patient's body or may be fabricated from an artificial,biocompatible material, such as GORTEX™ graft material, that will not berejected by the patient's body. Although arterial grafts are oftensurgically implanted within the thorax of a patient, they may also beemployed in other portions of the body. For example, an arteriovenousaccess graft or shunt is a specific type of graft employed tointerconnect an artery and a vein and is typically disposed just belowthe skin in a patient's arm so that it is readily accessible for use inhemodialysis, i.e., to couple a patient suffering from renal failure toa dialysis machine.

Once a graft is surgically implanted, it is difficult to monitor itscondition within a patient's body. Grafts often fail after a period oftime due to the build up of blocking deposits, thromboses, or tissuegrowth within the internal lumen of the graft or at its junctions withthe vessel in which it is inserted. It is estimated that the majority ofarteriovenous access grafts used for hemodialysis will fail within aboutone year following their installation. In many cases, steps may be takento restore full fluid flow through a graft that is becomingrestricted--but only if the preventive measures are taken before theproblem proceeds too far to be corrected without replacing the graft.Since it is generally not possible to determine the condition of flowthrough a graft without invasive surgery to inspect it, the procedurecommonly adopted in the case of access grafts is to simply replace thegraft each year, as a form of preventive maintenance. Clearly, it wouldbe preferable to monitor the condition of a graft without resorting toinvasive surgical procedures, so that the useful life of the graft canbe extended and so that problems that may arise due to the failure of agraft can be avoided.

The best indicators of the condition of a graft are the velocity andvolume of blood flowing through it. Fluid pressure at the distal andproximal ends of a graft (relative to the direction of blood flow) are afurther indication of a graft's condition. As the lumen through a graftgradually becomes occluded with fatty buildup or other deposits, thepressure differential across the graft will increase, the velocity ofblood in the lumen will decrease, and the flow of blood through thelumen will decrease. Each of these parameters thus serves as anindication of the condition of the graft and its viability to supportnecessary blood flow.

Ideally, it would be desirable to employ a graft--either natural orartificial--that includes means for monitoring the condition of fluidflow through the graft. The monitoring might occur continually or onlyperiodically, upon demand. The means used for monitoring the conditionof a graft should enable a physician to evaluate the parameters notedabove at a remote location outside the patient's body, without resortingto an invasive procedure. Further, the monitoring means should at leastperiodically be supplied power from an external source, since it isunlikely that a battery could provide the power required by sensors andcircuitry on the graft for an extended period of time.

Various techniques are known in the prior art for monitoring flow andvelocity of a fluid inside a blood vessel, but in each case, the devicesemployed for this purpose are intended for relatively short-term useimmediately following surgery and are not acceptable for the extendedperiod for monitoring fluid flow, as noted above. For example, one typeof volume flow measurement system described in U.S. Pat. No. 4,227,407uses two piezoelectric ultrasonic transducers that are alternatelyactivated to produce ultrasonic waves. The ultrasonic waves pass into avein or artery and are modified by the flow of blood in the vesselinterposed between the two transducers. When one transducer is activelytransmitting an ultrasonic wave, the other transducer serves as areceiver of the wave. The two transducers are oriented at an acute anglerelative to the longitudinal axis of the blood vessel, so that theultrasonic sound wave propagating through the blood vessel has acomponent in the direction (or opposite to the direction) of blood flowthrough the vessel. In an alternative embodiment disclosed in thispatent, the transducers are located on the same side of the bloodvessel, spaced apart along its longitudinal axis, and a reflective plateis disposed on the opposite side of the vessel, intermediate thepositions of the two transducers. An ultrasonic wave transmitted fromeither transducer passes through the blood vessel, is reflected from thereflective plate, and is received by the other transducer. Thedifference in the transit times for the sound waves transmitted from thetwo transducers (in both embodiments) is indicative of the flow throughthe blood vessel. If transducers used only extend over a small portionof the diameter of the vessel, the difference in transit time would beindicative of the velocity of blood flowing in the blood vessel.However, since the transducers shown in this prior art reference aresufficiently large so that the diameter of the blood vessel is fullyencompassed by the sound waves the transducers emit, the transit time isindicative of the flow of blood flowing through the vessel, i.e.,volumetric flow. The flow is thus determined without any considerationof the internal cross-sectional area of the blood vessel. While thisprior art apparatus is useful for monitoring blood flow (or velocity)through a blood vessel that is surgically exposed, the transducers aretoo large to be implanted within a patient's body and are unsuitable toremain attached to or be incorporated into a graft to monitor the fluidflow status of the graft. Also, to provide a good acoustic path betweenthe transducers and the adjacent surface of the vessel, it may well benecessary to apply the transducers against the surface of the vesselwith sufficient force to distort the wall of the vessel into the notchin the apparatus that is formed adjacent the sloping face of eachtransducer. Such distortion of the vessel may adversely affect theaccuracy of the measurements and is undesirable over an extended periodof time.

Another prior art approach for determining the velocity and/or flow ofblood in a vessel employs Doppler sensing using either a pulsed orcontinuous wave ultrasonic signal that is emitted at a defined anglerelative to the longitudinal axis of the blood vessel. If only a singletransducer is used, the angle must be accurately known, and any error inthe angle must be corrected. However, if a transmitting transducer isdisposed on one side of the blood vessel and a receiving transducer isdisposed on the opposite side of the blood vessel, angled so that theultrasonic beam reflected from the blood flowing through the vessel isdirected to the receiving transducer, an angle correction is notrequired.

Examples of apparatus for Doppler monitoring of blood flow are disclosedin U.S. Pat. Nos. 5,289,821 and 5,588,436. In the first of these twopatents, an ultrasonic transducer wire assembly is secured to a strip ofbiologically inert or absorbable material, which is wrapped around andin contact with a blood vessel to form a cuff, preferably disposeddownstream from an anastomosis of the vessel, such as may be performedduring microvascular surgery. The wire from the transducer exits thepatient's body through a slit and is coupled to ultrasonic processingmeans that determine the velocity of blood flowing through the vessel bythe Doppler processing of an ultrasonic wave that is transmitted by thetransducer and received as a reflection from the blood in the vessel.After monitoring the velocity of blood flow for about three to sevendays to determine if any thromboses has formed that would impede bloodflow, the wire and transducer can be pulled from the strip and removedfrom the body through a small incision, leaving the strip behind. Thisdevice is not usable for an extended period of time (much beyond sevendays), since the slit in the skin where the wires penetrate represents apathway for infection. Further, the patent teaches that the invention isprimarily intended for use on blood vessels close to the skin surface,such as those resulting from microvascular surgery on a patient's handand thus would be unusable for monitoring the fluid flow through graftsdeep within a patient's body.

In the second patent listed above, a Doppler scheme for determiningblood velocity in a vessel is disclosed, wherein an elongate sheath isprovided with a transducer head at its distal end. Two wires extendlongitudinally through the sheath to a transducer that is mountedpreferably at an angle of about 45° relative to the longitudinal axis ofthe sheath. A biocompatible material such as epoxy encases both thetransducer and the distal ends of the wires. This molded housing for thetransducer has a concave surface that fixes the transducer relative tothe blood vessel and provides a close fit to the surface of the bloodvessel to provide a path for ultrasonic sound waves produced by thetransducer to enter the blood vessel and for reflected waves to bedetected by the transducer. A mesh band is wrapped around thetransducer, and its ends are sutured together to hold the concavesurface of the material in contact with the outer surface of the bloodvessel. The band is made of VICRYL™ mesh or other absorbable/inertmaterial. A thread having ends that run inside and along thelongitudinal axis of the sheath secure the band to the distal end of thesheath. The proximal end of the sheath is preferably left extendingthrough the patient's skin after the device is installed to monitorblood velocity through a vessel in contact with the concave surface ofthe material at the distal end of the probe. After the measurements areconcluded (purportedly, after a maximum of up to 21 days), the thread iscut and pulled from its engagement with the band, so that thetransducer, wires, and sheath can be withdrawn, leaving the band inplace--possibly to be absorbed, depending on the material from which theband is fabricated.

Each of the Doppler devices discussed above is used to monitor thevelocity of blood through a vessel, and to the extent that thecross-sectional area of the vessel is assumed or known, the devicesenable flow to be estimated. However, neither prior art Doppler deviceis intended to monitor flow or velocity of blood for more than a fewdays. In addition, the elongate sheath used with the latter device isrelatively bulky and not suitable for installation where available spacearound the vessel or graft is limited. Both devices put the patient atrisk of infection, because at least the wires coupled to the transducermust extend from inside the patient's body through the skin, to anexternal monitoring system.

Another prior art technique for monitoring flow with a Doppler systemthat is more compact than the devices discussed above is based on asurface acoustic wave (SAW) transducer that couples a "leaky wave" intothe wall of a blood vessel. The SAW transducer includes pairs ofinterdigital electrodes fabricated on a piezoelectric substrate that isrelatively small, e.g., about 1.6 mm by 2.2 mm. This transducer isdescribed in a paper entitled "Miniature Doppler Probe Using aUnidirectional SAW Transducer" by T. Matsunaka and S. Yamashita. Toproduce a unidirectional interdigital SAW transducer, the drive signalapplied to half of the electrodes is phase shifted by 90° relative tothat applied to the other electrodes. The ultrasonic waves produced bythe device propagate primarily in only one direction at an angle, θ,thereby enabling the direction of fluid flow in a blood vessel to bedetermined. The wave that would normally be transmitted in the oppositedirection at an angle, -θ, is instead canceled by the interferencebetween the interdigital electrodes driven with signals that are phaseshifted relative to each other. This prior art reference states that thesignal produced by a prototype SAW transducer had a maximum amplitude ata radiation angle of about 54.5°, with a beam width of about 2.5 timesthe actual electrode width (one mm) and suggests that the beam widthmight be reduced by modifying the electrode layout to achieve a"focusing effect."

Several advantages of the interdigital electrode SAW transducer designrelative to the other devices available to measure flow and velocity ofblood through a graft are apparent. The interdigital SAW transducer issubstantially smaller in size than the prior art devices and requiresless energy to produce ultrasonic waves. Further, the beam width issubstantially wider than the physical size of the electrodes so that theapparatus can be made relatively small compared to the size of the beamthat it produces. In addition, unlike the single transducer apparatusshown in the prior art first discussed above, which produce both forwardand rearwardly directed waves that are affected by the velocity of bloodin either direction but cannot determine the direction of flow, theunidirectional SAW transducer is able to monitor fluid velocity anddetermine the direction of the fluid flow.

The prior art does not disclose an interdigital transducer that monitorstransit time. Instead, each of the interdigital transducers of the priorart SAW transducer discussed above produces a leaky SAW wave and employsthe Doppler effect to determine the velocity of blood in a vessel. Formonitoring velocity and flow through a graft, it would be preferable toemploy a transducer that is compact, like an interdigital SAWtransducer, but one that also has the ability to measure transit timeand thus flow, generally independent of any considerations of velocityprofile or cross-sectional area of the graft. This transducer should beimplantable, preferably built into or secured to the graft when thegraft is installed in a patient's body, supplied with electrical powerfrom a source outside the patient's body, without using wires thatpenetrate the dermal layer, and should also permit monitoring of theflow, velocity, and pressure of a fluid without use of wires that passthrough the skin. Currently, no compact prior art device is availablethat can remotely monitor flow and velocity parameters of an implantedgraft for long periods (e.g., for months or even years) of time.Further, none of the prior art devices is designed to be whollyimplanted, remotely monitored, and provided with power from a remotesource outside the patient's body.

SUMMARY OF THE INVENTION

In accord with the present invention, a graft adapted to be coupled intoa patient's vascular system is defined that includes a biocompatiblematerial formed into a generally cylindrical shape and having a circularwall defining a lumen extending along a longitudinal axis. The lumen isadapted to convey a fluid through the graft. A first transducer isdisposed within the wall of the graft and produces a signal indicativeof a parameter of the fluid that is flowing through the lumen. Coupledto the first transducer is an antenna coil for conveying the signal to apoint external to the patient's body, so that the signal is usable forevaluating a condition of the graft.

In one embodiment of the invention, the first transducer comprises afirst pressure sensor, and the signal that it produces is indicative ofa pressure of the fluid in the lumen. Preferably, the graft includes asecond pressure sensor that is disposed within the wall of the graft,also producing a signal indicative of the pressure of the fluid in thelumen. Since the first and second pressure sensors are disposed adjacentopposite ends of the graft and monitor the pressure of the fluid in thelumen at each end, they enable a differential pressure to be determinedas an indication of the condition of the graft.

In another embodiment, the first transducer includes a plurality ofelements formed on a piezoelectric substrate. When excited by a radiofrequency signal, the elements emit ultrasonic waves that propagate intothe lumen and are affected by the fluid flowing through the lumen. Thegraft also includes a receiver, which responds to the ultrasonic wavesby producing the signal indicative of the parameter. The receiver iscoupled to the coil so that the signal produced by the receiver isconveyed outside the patient's body. For this embodiment, the parameteris either a velocity or a flow of the fluid through the lumen of thegraft. The signal indicative of the parameter is determined as afunction of the fluid's effect on the ultrasonic waves within the lumen.

In one form of the invention that employs the ultrasonic waves, thereceiver comprises a second transducer that includes a plurality ofelements formed on a piezoelectric substrate. The second transducer isalso disposed within the wall of the graft and responds to the effectthat the fluid in the lumen has on the ultrasonic waves to produce thesignal indicative of the parameter. The first transducer and the secondtransducer are disposed on opposite sides of the graft, so that theultrasonic waves pass through the lumen when traveling between the twotransducers. The signal produced by the second transducer thus providesan indication of a transit time of the ultrasonic waves through thelumen. Preferably, the plurality of elements comprising the firsttransducer and the second transducer are sufficiently flexible toconform to a curved shape of the wall.

Also, the plurality of elements comprising the first transducer aredivided into a first portion and a second portion. The elementscomprising the first portion are interdigitally dispersed among elementscomprising the second portion and are adapted to couple to the radiofrequency signal in one polarity, while the elements comprising saidsecond portion are adapted to couple to the radio frequency signal in anopposite polarity. As a result, the ultrasonic waves produced by theelements comprising the second portion are phase shifted by about 180°relative to the ultrasonic waves produced by the elements comprising thefirst portion.

A phase shifter is included in one embodiment. For this embodiment, theelements comprising the first transducer are divided into four portionsarranged in an ordered array in which each successive element is from adifferent one of the four portions, taken in order. The radio frequencysignal is applied to the phase shifter, and a phase shifted signalproduced by the phase shifter is applied to at least two of eachsuccessive four elements to provide about a 90° phase difference betweenthe ultrasonic waves emitted by successive elements. Thus, theultrasonic waves that are emitted by the first transducer in onedirection are substantially canceled due to a destructive interference.

In another form of the invention, the first transducer and the secondtransducer are spaced apart from each other along a side of the graft. Areflector is disposed on an opposite side of the graft from the firsttransducer and generally opposite a point between the first transducerand the second transducer. The ultrasonic waves from the firsttransducer pass through the lumen and are reflected back toward thesecond transducer by the reflector.

Yet another form of the invention provides that the first transducer andthe second transducer alternately function as an emitter and as areceiver of the ultrasonic waves during successive time intervals. Theradio frequency signal is coupled to the plurality of elementscomprising the second transducer when the second transducer functions asthe emitter of the ultrasonic waves. During this time, the plurality ofelements comprising the first transducer are coupled to the coil andproduce the signal indicative of the parameter, in response to theultrasonic waves passing through the lumen. A multiplexer is used foralternately coupling the first and the second transducers to the radiofrequency signal and to the coil.

The frequency of the radio frequency signal is preferably controlled todetermine a beam angle along which the ultrasonic waves are emitted bythe first transducer.

To provide electrical power for energizing electrical components of thegraft, the coil is adapted to couple to a source of energy that isexternal to the patient's body. In one embodiment, the coil is disposedwithin the wall of the graft. In this form of the invention, the coilmay comprise a mesh of insulated wire formed in a plurality of loops.The coil may be generally saddle shaped, substantially conforming to acurvature of the wall about the longitudinal axis of the graft. Further,the coil is preferably adapted to electromagnetically couple to anexternal coil that is connected to the source of energy.

When the first transducer comprises the receiver, the radio frequencysignal is applied to the plurality of element as a pulse, causing theplurality of ultrasonic waves to be emitted as a pulse. The elementscomprising the first transducer then receive an echo of the pulse ofultrasonic waves that is reflected from the fluid. This echo is employedto determine the parameter based on a Doppler effect.

Another aspect of the present invention is directed to a system formonitoring a parameter indicative of a condition of a vessel relating toits ability to convey a fluid. The system includes a carrier band thatis adapted to couple about the vessel in close proximity to at least oneside of the vessel. A first transducer having a plurality of conformalelements is disposed in a spaced-apart array on the carrier band. Theplurality of conformal elements are sufficiently flexible and are shapedso that they are adapted to curve about the vessel, conforming to itsshape. The first transducer is adapted to couple to a radio frequencysignal and produces ultrasonic waves when excited by the radio frequencysignal. The ultrasonic waves are emitted from the plurality of conformalelements and are directed into an interior of the vessel. A receiver isdisposed to receive the ultrasonic waves after they have propagated atleast partially through the vessel. The receiver produces a signalindicative of an effect on the ultrasonic waves due to the fluid in thevessel. A coil is coupled to the receiver for transmitting the signalproduced by the receiver outside the patient's body.

Other aspects of the present invention are directed to methods thatinclude steps that are generally consistent with the functionsimplemented by components of the apparatus discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram showing a first embodiment of an implantedelectronic circuit for monitoring the status of a graft with a selectedtransducer from a plurality of transducers;

FIG. 2 is a block diagram of a second embodiment of an implantedelectronic circuit for monitoring the status of a graft using separatemultiplexers for transmit and receive functions;

FIG. 3 is a block diagram of a third embodiment of an implantedelectronic circuit for monitoring the status of a graft using separatemultiplexers and amplifiers for transmit and receive functions;

FIG. 4 is a block diagram of a fourth embodiment of an implantedelectronic circuit for monitoring the status of a graft that employs alocal transmitter to excite a selected transducer, and amodulator/transmitter for transmitting signals from the transducers;

FIG. 5 is a block diagram of a fifth embodiment of an implantedelectronic circuit for monitoring the status of a graft, where onetransducer is selected for transmitting and receiving, and amodulator/transmitter is used for transmitting the signal produced bythe receiving transducer;

FIG. 6 is a block diagram of a sixth embodiment of an implantedelectronic circuit for monitoring the status of a graft, wherein one ofa plurality of transducers is selectively coupled to amodulator/transmitter for transmitting a signal indicative of fluidpressure or other parameters;

FIG. 7 is a cross-sectional view of an implanted radio frequency (RF)coupling coil and an external coil;

FIG. 8 is a bottom view of the external coil shown in FIG. 7;

FIG. 9 is a cut-away side elevational view of an alternative externalcoil and a side elevational view of a graft, showing an integratedspiral RF coupling coil within the wall of the graft;

FIG. 10 is a cut-away side elevational view of a further embodiment ofan external coil and a side elevational view of a graft that includes asaddle shaped integrated RF coupling coil within the wall of the graft;

FIG. 11 is another embodiment of a woven spiral mesh RF coupling coilthat is integrally provided in a wall of a graft;

FIG. 12 is a cut-away view of a graft implanted at a substantial depthwithin a patient's body, showing an external coupling coil thatencompasses the portion of the patient's body in which the graft isdisposed;

FIG. 13 is a side elevational schematic view of a dual beam conformaltransducer array on a carrier band for use around a fluid carryingvessel, in accord with the present invention;

FIG. 14 is an end elevational view of the conformal transducer array ofFIG. 13, around a vessel;

FIG. 15 is a plan view of the conformal transducer array shown in FIGS.13 and 14, cut along a cut line to display the dual conformal arrays ina flat disposition;

FIG. 16 is an enlarged partial transverse cross-sectional view of thelayers comprising the conformal transducer array mounted on a carrierband that is disposed around a vessel wall;

FIG. 17 is an enlarged partial transverse cross-sectional view of thelayers comprising the conformal transducer array disposed within avessel wall of a synthetic graft;

FIG. 18 is an enlarged partial cross-sectional side view of a tiltedelement transducer array disposed within a wall of a synthetic graft;

FIG. 19A is an enlarged partial cross-sectional side view of a pressuretransducer disposed within the wall of a synthetic graft;

FIG. 19B is an enlarged side elevational view of a graft in which aredisposed a pair of pressure transducers;

FIG. 20A is a cross-sectional side view of a portion of a syntheticgraft in which are disposed transversely oriented transducers formonitoring flow using correlation measurements; and

FIG. 20B is a transverse cross-sectional view of the synthetic graftshown in FIG. 20A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is employed for monitoring the status of fluidflow through a vessel and is disclosed in connection with a preferredapplication in which the flow through a graft that has been implanted ina patient's vascular system is monitored. However, it is not intendedthat the invention be limited to that application. Although much of thefollowing disclosure relates to that medically related application ofthe invention, it is also contemplated that various aspects of thepresent invention are also applicable to monitoring the status of fluidflow through any type of vessel, including without limitation, fluidflow through a vessel employed in an industrial process.

Employment of the present invention in the above-noted medicalapplication addresses the problems noted above in the Background of theInvention. Specifically, if the status of fluid flow through a graftthat has been implanted in a patient's vascular system is to bemonitored for an extended period of time, the system used for thispurpose will very likely need to receive energy from an external sourceand must convey data indicating the status of fluid flow through theimplanted graft to an external monitoring device that is disposedoutside the patient's body. In many cases, it may be desirable tomonitor the status of flow through multiple grafts or at multiplelocations on a single graft. Thus, the data signal indicating the statusof fluid flow sensed by each separate transducer must be selected tomonitor the condition of fluid flow at each location of a transducer.However, in some cases, only a single transducer may be required tomonitor a parameter such as flow or velocity, which is indicative of theinternal condition of the graft.

FIG. 1 illustrates a first embodiment of an implanted electronics systemfor monitoring flow, applicable to the situation in which n transducersare included on one or more grafts implanted in the patient's body.Variations of the electronic circuit shown in FIG. 1 are discussed belowto accommodate specific conditions. In addition, other embodiments ofelectronic circuits are illustrated in FIGS. 2-6. These embodiments,like that of FIG. 1, are useful for providing power to transducers thatmonitor fluid flow or velocity through a graft, and for transmittingdata signals from the transducers outside a patient's body to anexternal remote monitoring console. Some of these circuits are bettersuited for certain types of measurements than others, and again,variations in the implanted electronic circuits are discussed below, asappropriate.

Each of the circuits shown in FIGS. 1-6 are intended to be implantedwithin the patient's body and left in place at least during the periodin which the flow conditions through one or more grafts are monitored todetermine the status of the graft. Although separate functional blocksare illustrated for different components of the implanted electroniccircuits in these Figures, any of the implanted electronic circuits canbe implemented in one or more application specific integrated circuits(ASICs) to minimize size and cost, which is particularly important whenthe electronic circuits are integral with a graft. The implantableelectronic circuits can be either included within the wall of a graft inthe case of a synthetic (i.e., man-made) graft, or may be simply affixedto or implanted adjacent to the graft for either man-made grafts ornatural grafts that comprise a portion of a vessel taken from adifferent location in the patient's circulatory system.

Each of the circuits shown in FIGS. 1-6 includes an RF coupling coil 30,which is connected through lines 34 and 36 to an RF-to-DC power supply32. This power supply rectifies and filters an RF excitation signalsupplied from an external source to RF coupling coil 30, providing anappropriate voltage DC power signal for the other components of thecircuits illustrated in these Figures. In the simplest case, theRF-to-DC power supply would only require rectifiers and filters asappropriate to provide any needed positive and negative supply voltages,+V_(s) and -V_(s). However, it is also contemplated that the powersupply may provide for a DC-to-DC conversion capability in the eventthat the electromagnetic signal coupled into RF coupling coil 30 is tooweak to provide the required level of DC voltage for any component. Thisconversion capability would increase the lower voltage produced by thedirect coupling of the external RF excitation signal received by the RFcoupling coil, to a higher DC voltage. Details of the RF-to-DC powersupply are not shown, since such devices are well known to those ofordinary skill in constructing power supplies. It is also contemplatedthat it may be necessary to limit the maximum amplitude of the RF inputsignal to the RF-to-DC power supply to protect it or so that excessiveDC supply voltages are not provided to the other components.Alternatively, each component that must be provided with a limited DCvoltage supply may include a voltage limiting component, such as a zenerdiode or voltage regulator (neither shown).

The RF-to-DC power supply may include a battery or a capacitor forstoring energy so that it need not be energized when monitoring the flowstatus, or at least, should include sufficient storage capability for atleast one cycle of receiving energy and transmitting graft statusindicative data outside the patient's body. Although a storage batterycan be included, size limitations may prohibit any significant storagecapacity. Instead, a relatively small capacitor could provide therequired storage capability. Neither a battery or power storagecapacitor are illustrated in the Figures, since they are well known tothose of ordinary skill and are only optional.

An additional element that is common to each of the circuits shown inFIGS. 1-6 is an RF decode section 40, which is used for generatingcontrol signals that are responsive to information encoded in theexternal RF excitation signal received by RF coupling coil 30. Thisinformation can be superimposed on the RF excitation signal, e.g., byamplitude or frequency modulating the signal received.

In regard to the circuits shown in FIGS. 1-3, the RF excitationfrequency is the same as the frequency used to excite a selectedultrasonic transducer to produce an ultrasonic wave that propagatesthrough a vessel or graft being monitored, and for conveying data from atransducer that receives the ultrasonic waves. This approach generallysimplifies the implantable electronic circuitry but may not provideoptimal performance. Therefore, FIGS. 4 and 5 disclose implantableelectronic circuitry in which the RF excitation frequency used toprovide power to the RF-to-DC power supply and to provide controlsignals to RF decode section 40 is decoupled from the frequency that isused for exciting the transducers and modulating the data that theyprovide for transmission to a point outside the patient's body.

Details of the Implantable Electronic Circuits

Referring now to FIG. 1, line 36 from RF coupling coil 30 is connectedto a multiplexer (MUX) 38 to convey signals from a selected one of aplurality of n transducers 44-46 that are coupled to the MUX. To selectthe transducer that will provide the data signal related to the statusof flow through the graft being monitored, RF decode section 40 providesa control signal to MUX 38 through MUX control lines 42. The controlsignal causes the MUX to select a specific transducer that is to beexcited by the RF signal received by RF coupling coil 30 and further,causes the MUX to select the transducer that will provide the datasignal for transmission outside the patient's body via RF coupling coil30.

In addition to ultrasonic transducers, the implantable electroniccircuit shown in FIG. 1 can also be used in connection with pressuretransducers. For ultrasonic transducers, the circuit is perhaps moreapplicable to the Doppler type for use in monitoring fluid velocitythrough a graft. If a single-vessel pulse Doppler transducer is used,the same transducer can be used for both transmission and reception ofthe ultrasonic wave, thereby eliminating the need for MUX 38. In theevent that the transducers shown in FIG. 1 are used for transit timeflow measurements, it will normally be necessary to use MUX 38 to switchbetween the transducer used for transmitting the ultrasonic wave andthat used to receive the ultrasonic wave, which may present someproblems in connection with switching speed, power consumption, andswitching transient recovery.

For a single-vessel transit time measurement, a pair of opposedtransducers that are disposed on opposite sides of the graft aretypically used. In order to acquire bi-directional fluid flow data, thedirection of the ultrasound wave propagation must be known, i.e., thedirection in which the ultrasound wave propagates relative to thedirection of fluid flow through the vessel. In this case, MUX 38 isrequired. However, for single-vessel applications in which the fluidflow is in a single known direction, the transducers that are disposedon opposite sides of the graft can be electrically connected in parallelor in series, eliminating any requirement for MUX 38. The RF-to-DC powersupply and RF decode sections could also then be eliminated, since theretarded and advanced transit time signals would be superimposed on thesame RF waveform transmitted by RF coupling coil 30 outside thepatient's body. Although this modification to the implantable electroniccircuit shown in FIG. 1 would not permit the direction of fluid flowthrough a graft to be determined, the retarded and advanced transit timesignals would interfere as they propagate in time, and theirinterference can be used to estimate the magnitude of fluid flow throughthe graft.

In FIG. 2, an implantable electronic circuit is shown that uses atransmit multiplexer (TX MUX) 50 and a receive multiplexer (RX MUX) 54.In addition, a transmit (TX) switch 48 and a receive (RX) switch 52couple line 36 to the TX MUX 50 and RX MUX 54, respectively. RF decodesection 40 responds to instructions on the signal received from outsidethe patient's body by producing a corresponding MUX control signal thatis conveyed to TX MUX 50 and RX MUX 54 over MUX control lines 56 toselect the desired transducers.

When ultrasonic signals are being transmitted by one of the selectedtransducers 1 through n, a TX switch 48 couples the RF excitation signalreceived by RF coupling coil 30 to the transducer that is transmittingthe ultrasonic signal, which is selected by TX MUX 50. The TX switch isset up to pass excitation signals to the selected transducer only if thesignals are above a predetermined voltage level, for example, 0.7 volts.Signals below that predetermined voltage level are blocked by the TXswitch. Similarly, a RX switch 52 connects the transducer selected by RXMUX 54 to RF coil 30 and passes only signals that are below thepredetermined voltage level, blocking signals above that level.Accordingly, the RF signal used to excite a first transducer selected byTX MUX 50 passes through TX switch 48 and the lower amplitude signalproduced by a second transducer selected by RX MUX 54 in response to theultrasonic signal transmitted through the graft is conveyed through RXMUX 54 and RX switch 52 and transmitted outside the patient's bodythrough RF coil 30.

The implantable electronic circuit shown in FIG. 3 is similar to that ofFIG. 2, but it includes a transmit amplifier (TX AMP) 58 interposedbetween TX switch 48 and TX MUX 50, and a receive amplifier (RX AMP) 60interposed between RX MUX 54 and RX switch 52. TX AMP 58 amplifies theexcitation signal applied to the transducer selected by TX MUX 50 forproducing the ultrasonic wave that is propagated through a graft.Similarly, RX AMP 60 amplifies the signal produced by the transducerselected by RX MUX 54 before providing the signal to the RX switch fortransmission outside the patient's body. Again, the circuit shown inFIG. 3 is most applicable to transit time flow measurements and employsthe same frequency for both the RF excitation signal that supplies powerto RF-to-DC power supply 32 and the signal applied to a selected one oftransducers 44-46 to generate the ultrasonic wave propagating throughthe graft.

In contrast to the implantable electronic circuits shown in FIGS. 1-3,the circuit shown in FIG. 4 enables the RF excitation frequency appliedto RF-to-DC power supply 32 to be decoupled from the signal applied toexcite any selected one of transducers 44-46. Similarly, the signalproduced by the transducer receiving the ultrasonic waves propagatingthrough the graft is at a different frequency than the RF excitationfrequency. In FIG. 4, a transmitter (XMTR) 62 and a receivemodulator/transmitter (RX MOD/XMTR) 64 are coupled to and controlled byan RF decode/control section 66. The RF decode/control sectiondetermines when the excitation frequency is generated for application toa selected transmit transducer and when the signal produced by thetransducer selected to receive the ultrasonic wave is used formodulating the RF signal applied to RF coupling coil 30. An advantage ofthis approach is that the RF power delivered to RF coupling coil 30 isat an optimal frequency for penetration through the patient's body,thereby improving the efficacy with which the RF energy couples to aspecific depth and location within the body. Another reason is forsatisfying any requirements for selecting a particular frequency tocomply with radio frequency allocation bands for medical equipment.Similarly, the frequency applied to any selected transducers 44 and 46to stimulate their production of ultrasonic waves can be optimal forthat purpose. Assuming that the two frequency bands, i.e., the RFexcitation frequency band for the signal applied to the power supply andthe frequency band applied to excite the transducers, are sufficientlyseparated, the RF power delivery can occur simultaneously with theexcitation of a selected transducer and the reception of the ultrasonicwaves by another selected transducer. Accordingly, more RF power can becoupled into the system from the external source than in the implantableelectronic circuits shown in FIGS. 1-3.

The control signals that are supplied to RF decode/control section 66via RF coupling coil 30 can be conveyed using nearly any kind ofmodulation scheme, e.g., by modulating the RF excitation that powers thedevice, or by sending a control signal on a separate and distinct RFfrequency. Also, the signals that are received from the transducer inresponse to the ultrasonic wave that is propagated through the graft canbe transmitted through the RF coupling coil at a different frequencythan the incoming excitation frequency, thereby eliminating interferencebetween the power supply and data signal transmission functions.

The implantable electronic circuit shown in FIG. 4 is most applicable totransit time flow measurements in which pairs of transducers areselected for transmitting and receiving the ultrasonic wave thatpropagates through the one or more grafts on which the transducers areinstalled. RF decode/control section 66 can be employed to control TXMUX 50 and RX MUX 68 to interchange the transducers used fortransmission and reception of the ultrasonic wave on successive pulses.Using this technique, the direction of the ultrasonic wave propagationthrough the graft is changed on alternating pulses of ultrasonic waves,enabling transit time difference information to be gathered withoutrequiring further multiplexer programming information to be transmittedbetween successive ultrasonic wave pulses. This approach greatlyimproves the data gathering efficiency of the implantable electroniccircuit shown in FIG. 4 compared to the previously described implantableelectronic circuits of FIGS. 1-3.

To further improve the implantable electronic circuit shown in FIG. 4for use in sensing fluid velocity through a graft using a Dopplertechnique, the modification shown in FIG. 5 is made. In the latterimplantable electronic circuit, a TX/RX switch 72 is added so that thecircuit transmits and receives through the same transducer. As a result,separate transmit and receive multiplexers are not required. Instead,MUX 38 is used to select the specific transducer for receiving the RFexcitation signal produced by XMTR 62 so that the transducer produces anultrasonic wave and then receives the echo from fluid flowing throughthe graft to produce a receive data signal that is output through RXMOD/XMTR 64. TX/RX switch 72 prevents the signal applied by TX AMP 58from overdriving the input to RX AMP 60, effectively isolating the RXAMP during the time that the RF signal is applied to the transducer toexcite it so that it produces the ultrasonic wave. However, the echosignal received by the transducer is allowed to reach RX AMP 60 whenTX/RX switch 68 changes state (from transmit to receive). Generally, theimplantable electronic circuit shown in FIG. 5 has the same benefits asdescribed above in connection with the circuit shown in FIG. 4. RFdecode/control section 66 responds to the information received fromoutside the patient's body that determines which one of transducers44-46 is selected at any given time by producing an appropriate MUXcontrol signal that is supplied to MUX 38 over MUX control lines 56.

It is also contemplated that RF decode/control section 66 may cause MUX38 to select a different transducer for producing/receiving theultrasonic waves after a predefined number of transmit/receive cycleshave elapsed. For example, a different transducer may be selected aftereight cycles have been implemented to transmit an ultrasonic wave intothe graft and to receive back the echoes from the fluid flowing throughthe graft. By collecting data related to the status of flow through oneor more grafts in this manner, it becomes unnecessary to sendprogramming information to RF decode/control section 66 after each cycleof a transmission of the ultrasonic wave into the fluid in the graft andreception of the echo. By carrying out a predefined number oftransmit/receive cycles for a given transducer that has been selected byMUX 38 and averaging the results, a more accurate estimate of fluidvelocity through the graft can be obtained than by using only a singletransmission and reception of an ultrasonic wave. Since the signalrequired to instruct RF decode/control section 66 to change to the nexttransducer is only required after the predefined number of cycles hasbeen completed, the data gathering efficiency of the implantedelectronic circuit is improved.

Although transducers 44-46 that are shown in FIGS. 1-5 need not beultrasonic transducers, FIG. 6 illustrates an implantable electroniccircuit that is particularly applicable for use with transducers 44-46comprising pressure sensors. For example, such pressure sensors could bedisposed within the wall of a synthetic graft to sense the pressure offluid flowing through the graft at one or more points. MUX 38 is usedfor selecting a specific pressure transducer to provide a data signalthat is transmitted to the outside environment via RF coupling coil 30.In this circuit shown in FIG. 6, a modulator/transmitter (MOD/XMTR) 70receives the signal from the transducer selected by MUX 38 in responseto the MUX selection signal provided over MUX control lines 56 from RFdecode/control section 66 and using the signal, modulates an RF signalthat is supplied to RF coupling coil 30. The RF signal transmitted bycoupling coil 30 thus conveys the data signal indicating pressure sensedby the selected transducer. In many cases, it will be preferable tomonitor the pressure at the distal and proximal ends of a graft in orderto enable the differential pressure between these ends to be determined.This differential pressure is indicative of the extent to whichthromboses or other source of blockage in the interior lumen of thegraft is impeding fluid flowing through the lumen. In most cases,parameters such as fluid flow or velocity are better indicators of thestatus of flow through the graft.

RF Coupling Coil and External Coil Embodiments

FIGS. 7-12 illustrate details of several different embodiments for theRF coupling coil that is implanted within a patient's body for receivingRF energy to provide power for the implanted electronic circuitsdiscussed above and for transmitting data relating to the condition offlow through one or more grafts that have been installed within thepatient's vascular system. Optimization of RF coupling between the RFcoupling coil that is implanted and the external coil is partiallydependent upon the propagation characteristics of the human body. Sincethe body tissue is largely comprised of water, the relative dielectricconstant of flesh is approximately equal to that of water, i.e., about80. Also, the permeability of tissue comprising a body is approximatelyequal to one, i.e., about that of free space. The velocity ofpropagation of an RF signal through the body is proportional to theinverse square root of the dielectric constant and is therefore about11% of the velocity of the signal in free space. This lower velocityreduces the wavelength of the RF signal by an equivalent factor.Accordingly, the wavelength of the RF signal transferred between theimplanted RF coupling coil and the external coil would be a designconsideration if the separation distance between the two isapproximately equal to or greater than one-quarter wavelength. However,at the frequencies that are of greatest interest in the presentinvention, one-quarter wavelength of the RF coupling signal should besubstantially greater than the separation distance between the twocoils.

The penetration of RF fields in the human body has been studiedextensively in conjunction with magnetic resonance imaging (MRI)systems. RF attenuation increases with frequency, but frequencies ashigh as 63 MHz are routinely used for whole-body imaging, although someattenuation is observed at the center of the torso at the upper end ofthe frequency range. In addition, MRI safety studies have also provideda basis for determining safe operating limits for the RF excitation thatdefine the amplitude, which can be safely applied without harm to thepatient.

It is contemplated that for graft implants placed deep within theabdomen of a patient, RF excitation and frequencies used forcommunicating data related to the fluid flow through a graft can be upto about 40 MHz, although higher frequencies up to as much as 100 MHzmay be feasible. At 40 MHz, the wavelength of the RF excitation signalin tissue is about 82 cm, which is just that point where wavelengthconsiderations become an important consideration. For shallow implants,RF excitation at a much higher frequency may be feasible. For example,access grafts that are used for hemodialysis are typically only about 5mm beneath the surface of the skin, in the forearm of the patient. Toprovide energy to the implanted electronic circuit and to receive datafrom transducers associated with such grafts, frequencies in the rangeof a few hundred MHz may be useful. The dielectric properties of tissuehave been studied to at least 10 GHz by R. Pethig, Dielectric andElectronic Properties of Biological Materials, Wiley Press, Chichester,1979 (Chapter 7). Based on this study, no penetration problems areanticipated in the frequency range of interest. The dielectric constantof tissue decreases to about 60 at a frequency of 100 MHz and is about50 at 1 GHz, but this parameter has little effect on power/data signalcoupling.

In FIG. 7, an RF coupling coil 30' is disposed opposite a correspondingexternal coil 90. RF coupling coil 30' includes a toroidal coil 82 thatis wound in the hollow center channel of a toroidal shaped core 84. Core84 and toroidal coil 82 are contained within a biocompatible housing 80that also provides RF shielding around the coil except where it liesopposite to external coil 90. External coil 90 is of similar design,including a toroidal coil 92 disposed within the hollow center portionof a toroidal shaped core 94. A housing 96 comprising an RF shieldencloses much of the toroidal coil and core. A cable 98 conveys signalsto and from an external power supply and patient monitoring console 100,which is shown in FIG. 8.

The external coil and RF coupling coil shown in FIGS. 7 and 8 representone embodiment used for coupling electrical energy and conveying datasignals across a skin interface 102 for applications in which the RFcoupling coil is implanted relatively close to the surface of the skin.For example, RF coupling coil 30' and external coil 90 would provide thecoupling required for a system used to monitor coronary artery bypassgrafts (CABG). During CABG surgery, a patient's chest is opened, makingit relatively straightforward to implant RF coupling coil 30' beneaththe skin as the chest is being closed at the conclusion of this surgicalprocedure.

Although the external core and internal core need not be identical insize and shape, it is generally true that coupling will be optimal ifthe annular surfaces of the two cores are of approximately the samedimensions and if the core halves are aligned. By observing the strengthof the signal transmitted from RF coupling coil 30', it should bepossible to position external coil 90 in proper alignment with theimplanted coil so that the amplitude of the signal is maximized.

To function as a transformer core, the material used must have arelatively high magnetic permeability, at least greater than one.Although ferrite is commonly used for core materials, sintered powderediron and other alloys can also be used. Since the choice of materialsfor the cores of the RF coupling coil and the external coil based on themagnetic characteristics of such materials are generally well understoodby those of ordinary skill in the art, further details need not beprovided herein to provide an enabling disclosure of the presentinvention.

Housing 96 on external coil 90 provides RF shielding againstelectromagnetic interference (EMI). Housing 96 is preferably conductive,grounded, and surrounds the external coil except where the face of core84 is opposite core 94 of the implanted coupling coil. The RF shieldcomprising housing 96 also includes a split annular ring 116, which isattached to the internal shield (not separately shown) at cable 98. Asimilar split annular ring 86 is provided on RF coupling coil 30'covering toroidal core 82. Split annular rings 86 and 116 are used sothat a shorted turn is avoided that would otherwise tend to attenuate acoupling between the external coil and RF coupling coil. The housing ofthe implanted coupling coil is connected to the shield on the cable, andthe shield is connected to a shield on the graft. Inside power supplyand patient monitoring console 100, the shield on cable 98 is connectedto ground. The RF shields on both the external coil and the RF couplingcoil that is implanted within the patient, along with the shieldsprovided around the transducers (described below) minimize external EMIradiation due to the use of the present invention within a patient'sbody.

Referring now to FIG. 9, an RF coupling coil 30" is shown that comprisesa plurality of spiral conductor coils 108 disposed within the wall of agraft 106. Although the drawing shows only a single layer of spiralcoils 108, it is contemplated that a plurality of layers of such coilsmay be used and that the spacing between the spiral coils may besubstantially closer than illustrated in the Figure. Coupling coil 30"is connected to an electronics assembly 110 that may include any of theimplantable electronic circuits shown in FIGS. 1-6. Not shown in FIG. 9are the transducers that are provided within the wall of or on theexternal surface of graft 106.

RF coupling coil 30" would typically be used in connection with a graftthat is disposed relatively close to the outer surface of the patient'sbody, for example, within tissue 104 immediately below a dermal layer102. In this disposition, the RF coupling coil more readily couples toan external coil 90'. External coil 90' shown in FIG. 9 has a generallyC-shaped core 94' about which is coiled a plurality of turns 92'. Leads98 pass through a housing 96' that comprises an RF shield and connectthe external coil to a power supply and monitoring system (not shown).Lines of magnetic flux 112 intersect spiral coils 108 on RF couplingcoil 30' to provide electrical power for energizing electronics assembly110. Similarly, RF coupling coil 30" generates an EMI concentrated alongthe longitudinal axis of graft 106 that is sensed by external coil 90'to convey data indicating the flow status of the fluid through graft 106to the power supply and monitoring system.

Core 94' of external coil 90' is preferably fabricated of a ferrite corematerial, or other suitable alloy. The number of coils 92', the size ofthe wire, size of the core, and other parameters can be determined for aparticular frequency of operation using conventional transformer designcriteria, by one of ordinary skill in the art.

In FIG. 10, an RF coupling coil 30'" is illustrated that comprises aplurality of generally saddle shaped coils 114 disposed within the wallof graft 106. Again, the RF coupling coil is coupled to electronicsassembly 110. Although only a single layer of saddle shaped coils 114 isillustrated, it is contemplated that a plurality of such interconnectedlayers could be provided within the wall of the graft.

For use with RF coupling coil 30'", an external coil 90" is providedthat includes a plurality of coils 92" wrapped around a central portionof a generally E-shaped core 94". Lines of electromagnetic flux are thusproduced between the central leg and each of the end legs of core 94".It will therefore be apparent that this embodiment of the RF couplingcoil and of the external coil achieve optimum coupling when the distanceseparating the two is minimal. Therefore, RF coupling coil 30'" andexternal coil 90" are best used in applications where graft 106 isdisposed relatively close to dermal layer 102 so that tissue 104separating the graft from external coil 90" is only a few centimetersthick. For example, this embodiment of the RF coupling coil and externalcoil is applicable for use with access grafts implanted just beneath theskin on the patient's forearm. Maximal coupling is achieved when thelongitudinal axis of external coil 90" is aligned with the longitudinalaxis of graft 106.

A further embodiment of an RF coupling coil 130 that is disposed withina graft 144 is shown in FIG. 11. RF coupling coil 130 comprises a wovenmesh 132 fabricated from insulated wire so that overlapping segments ofthe mesh do not electrically connect in the center of the graft. At eachend of the RF coupling coil, the wires comprising wire mesh 132 areelectrically coupled together, producing a multi-turn coil. If each wirecomprising the mesh passes around the central axis of the graft throughm degrees, and if there are a total of n such wires, then the equivalentnumber of turns in coupling coil 130 is equal to n×m÷360. Leads 136 and138 convey signals to and from nodes 134, connecting the wire mesh toelectronics assembly 110. A biocompatible coating 142 surrounds themesh, protecting it from contact with bodily fluids.

In those cases where grafts are implanted relatively deep inside thepatient's body, at some distance from the surface of the patient's skin,an alternative external coil 154 can be employed, generally as shown inFIG. 12. In this example, an artery 152 includes a graft 144 comprisingRF coupling coil 130, which is disposed within a thigh 150 of thepatient. To couple with RF coupling coil 130, RF coupling coil 154includes a plurality of turns 156 sufficient in diameter to encompassthigh 150. An RF shield 160 encloses the outer extent of RF couplingcoil 154, so that RF energy radiates only from the inner portion ofcoils 156. A lead 158 couples RF coupling coil 154 to the power supplyand monitoring console (not shown in this Figure). RF coupling coil 154can be made sufficiently large to encompass the portion of the body inwhich the implanted graft is disposed such as the torso, another limb ofthe patient, or the neck of the patient. Coupling is maximized betweenexternal coil 154 and coupling coil 130 (or other RF coupling coil) usedon the graft when the central axes of both the RF coupling coil and theexternal antenna are coaxially aligned and when the implanted graft isgenerally near the center of the external coil. Coupling between the twocoils decreases with increasing separation and begins to degrade whenthe implanted graft is more than one external coil radius away from thecenter point of the external coil. In addition, coupling is minimizedwhen the central axes of the two coils are perpendicular.

Description of the Ultrasonic Transducer Arrays

An ultrasonic transducer for monitoring flow or fluid velocity through agraft should be relatively compact if it is to be mounted adjacent anatural graft or included in the wall of a synthetic graft. Typically, aprior art ultrasonic transducer includes an element comprising a planarslab of a piezoelectric material having conductive electrodes disposedon opposite sides thereof. Since such elements are relatively planar,they do not conform to the circular cross-sectional shape of a graft andtherefore, are not compact or appropriate for use with a graft that isimplanted within a patient's body and which is intended to be left inplace for an extended period of time.

FIG. 13 shows an embodiment of an extremely low profile ultrasonictransducer comprising a conformal array 174a disposed on opposite sidesof graft from a conformal array 174b. Ideally, the conformal arraycomprises a piezoelectric plastic used as a transduction material andhaving sufficient flexibility to allow the transducer elements to bewrapped around a wall 168 of a vessel 170. Such flexible piezoelectricplastic materials are readily available. It should be noted that vessel170 may comprise either a natural or synthetic graft, or may instead besimply a part of the patient's vascular system. However, the compact,low profile aspect of the conformal transducer array makes it ideallysuited for other applications outside the medical field. It is thereforecontemplated that the conformal array ultrasonic transducer shown inFIGS. 13, 14, and 15 may alternatively be used in other commercial andindustrial applications in which space around a vessel wall is at apremium and there is a need to monitor flow and/or velocity of a fluidthrough the vessel. Thus, the conformal array transducer may be used tomonitor fluid flow or velocity through a plastic or metal pipe or tube.Furthermore, it can be used for either transit time or Dopplermeasurements. When used for transit time measurements, as shown in FIGS.13 and 14, conformal arrays 174a and 174b are disposed generally onopposite sides of the vessel and encompass much of the circumference ofthe vessel.

However, when a pulsed Doppler measurement is made using the conformalarray transducer, only a single such transducer is required, since itfirst produces an ultrasonic wave that is transmitted into the vesseland then receives an echo reflected back from the fluid flowing throughthe vessel. If used for continuous wave (CW) Doppler measurements, thepair of conformal array transducers disposed on opposite sides of thevessel are again needed, one transducer serving as a transmitter and theother as a receiver. In each case, it is presumed that the fluid has anon-zero velocity component directed along an ultrasonic beam axis ofthe ultrasonic wave produced by the conformal array transducer servingas a transmitter.

Conformal arrays 174a and 174b shown in FIGS. 13-15 produce ultrasonicbeams 178 that are tilted relative to the transverse direction acrossvessel 170 in substantially equal but opposite angles with respect tothe longitudinal axis of the vessel. Since dual beam transit timemeasurements are implemented by conformal arrays 174a and 174b, theresults are self-compensating for tilt angle errors. This form ofself-compensation is only required where the alignment of the conformalarray relative to the longitudinal axis of the vessel may be imperfect.For example, such imperfections are likely to occur when the conformalarrays are used in connection with monitoring the status of fluid flowthrough grafts or vascular vessels within a patient's body, since thegrafts and vessels are not rigid and frequently are not straight--evenwithin the limited length of the conformal array. For transit timemeasurements made on vessels wherein the alignment of the transducerrelative to the longitudinal axis of the vessel is well known, anopposed pair of conformal arrays disposed on opposite sides of thevessel is sufficient so that the added complexity of the dual beamtransducer geometry is not required for self compensation.

In the case of pulsed Doppler velocity measurements, a single transducerwould again likely be adequate so long as the alignment of thetransducer to the vessel is accurately controlled. If the alignment ofthe conformal array transducer is not controlled or not well known, asecond such transducer can be used to gather velocity data along asecond beam axis using pulsed Doppler velocity measurements. Assumingthat the second axis is tilted in an equal but opposite direction as thefirst axis, the Doppler measurements made by the two conformal arraytransducers should be self-compensating for tilt errors. In this case,the second conformal array transducer could be mounted on the same or onan opposite side of the vessel from that where the first conformal arraytransducer is mounted to implement the Doppler measurements.

For CW or pseudo-CW Doppler velocity measurements (in which a relativelylong duration pulse of ultrasonic waves is produced), the transit signalis applied for a sufficiently long period so that a second transducer isneeded to receive the echo signals. In this case, a single set ofdiametrically opposed conformal array transducers can be used.

As perhaps best illustrated in FIG. 14, conformal array transducers 174aand 174b need not wrap entirely around vessel 170. In the illustratedembodiment, the conformal array transducers each span an arc ofapproximately 120° around the longitudinal axis of the vessel (i.e.,about the center of the circular vessel as shown in FIG. 14). Thisgeometry produces a measurement zone through which ultrasonic beams 178propagate that is nominally equal to about 87% of the vessel outerdiameter. Since vessel wall 168 has a finite thickness, the actualmeasurement zone (within the lumen of the vessel) exceeds approximately90% of the vessel internal diameter. If used for Doppler velocitymeasurements, it is contemplated that the conformal array transducerneed cover only a central portion of the vessel. As a result, the spanof the conformal array transducer can be reduced from about 120° tosomething within the range from about 60° to about 90°.

To produce a wide, uniform ultrasound beam such as that needed fortransit time measurements of flow, the conformal array transducer mustproduce ultrasonic waves having a wave front characterized by asubstantially uniform amplitude and phase. As shown in FIG. 13, lateralprojections through each of a plurality of transducer elementscomprising the conformal array transducers are indicated by straightlines 176. These straight lines indicate the centers of the transducerelements and are perpendicular to the axis of propagation of waves 178(represented by bidirectional arrows directed along the axes ofpropagation of the ultrasonic waves). In the preferred embodiment, thespacing between the element centers, i.e., between straight lines 176,is approximately equal to a phase angle of 90° at the transducer'sexcitation frequency. Thus, starting at the top of FIG. 13 and workingdownwardly, transducer elements disposed along each of the displayedstraight lines produce acoustic waves that are successively delayed by90°, or one-quarter wavelength in the fluid medium through which theultrasonic waves propagate. For tissue, a sound velocity of 1,540meters/second is normally assumed, so that the physical spacing of theprojected straight lines would typically be defined by the following:

    Projected Spacing in millimeters=1.54/(4*F.sub.0)

where F₀ is equal to the center frequency in MHz. If zero degrees isassigned to the top-most element of conformal array 174a, the nextelement would operate at -90° relative to the top element, followed byan element operating at -180°, and then one operating at -270°, andfinally by an element operating at 0° relative to the top electrode.Thus, conformal array 174a produces a succession of ultrasonic wavesspaced apart by a 90° space shift, thereby achieving a desired phaseuniformity across the transducer.

Amplitude uniformity can be achieved in the ultrasonic wave front by"shaving" the elements of the conformal array. Although shaving could beachieved in a variety of ways, the preferred embodiment controls shavingby varying the area of each element.

Conformal array transducers 174a and 174b are carried on a band 172preferably made from the piezoelectric plastic material used for theelement substrate, which is sized to fit snugly around an outer surfaceof vessel 170. Band 172 is intended to position the conformal arraytransducers in acoustic contact with vessel wall 168. Such contactassures that the ultrasonic waves produced by the element of theconformal array are conveyed through the vessel wall and into the fluidflowing through the interior of the vessel. Preferably, thepiezoelectric plastic comprising band 172 is fabricated from a materialsuch as polyvinylidene fluoride (PVDF), poly(vinyl cyanide-vinylacetate) copolymer (P(VCN/VAc), or poly(vinylidenefluoride-trifluoroethylene) copolymer (P(VDF-TrFE)). Preferably,P(VDF-TrFE) is used because of its superior piezoelectric coupling andrelatively lower losses.

Referring now to FIG. 15, further details of the conformal arraytransducers are illustrated. In this embodiment, alternating elements ofthe conformal array produce ultrasonic waves differing by 90°. In theview shown in FIG. 15, a cut line 175 intersects the lateral center ofconformal array 174b. In practice, any cut would more likely extendthrough band 172 at a point approximately midway between conformal array174a and conformal array 174b. If band 172 must be cut in order to wrapthe band around a vessel 170, i.e., when it is not possible or practicalto slip band 172 over the vessel uncut, the elements comprising theconformal array transducers need not be interrupted or damaged.Electrodes comprising each element of the conformal array can bephotolithographically generated on the piezoelectric plastic substratecomprising band 172. Alternatively, the elements can be formed on anon-piezoelectric material comprising band 172, and then the materialwith the elements formed thereon can be bonded to a piezoelectricsubstrate in each area where a conformal array transducer element isdisposed. In this latter embodiment, it is contemplated that a flexcircuit material such as a polyimide could be employed for band 172, andthat conventional photolithographic processing methods might be used tofabricate the conformal array transducer circuitry on the band. Further,the centers of alternating conformal array elements are coupled togetherelectrically via conductors 180 (shown as dash lines) in FIG. 15. Notshown in FIGS. 13-15 are the leads that extend from an electronicsassembly used to drive the conformal array. Any of the implantableelectronic circuits shown in FIGS. 1-5 could be used for the electronicsassembly.

The pattern of elements comprising each of the conformal arraytransducers and the boundary of each conformal array (top and bottom asshown in FIG. 15), define sinusoidal segments. The period of the sinewave from which these sinusoidal segments are derived is approximatelyequal to the circumference of band 172. Further, the amplitude of thatsine wave generally depends on the desired beam angle relative to thelongitudinal axis of vessel 170. For the sinusoidal segment employed foreach electrode, the amplitude is defined by:

    Amplitude=D*tanΘ

Similarly, the amplitude of the sinusoidal segment defining the boundaryof each conformal array is defined by:

    Amplitude=D/(tanΘ)

where Θ is equal to the angle between the longitudinal axis of thevessel and the ultrasound beam axis and D is equal to the externaldiameter of the vessel. Accordingly, it should be apparent that onesinusoidal template could be used to draw all of the transducer elementsand a second sinusoidal template (differing only in amplitude from thefirst) could be used to draw the boundary of each conformal arraytransducer. The transducer elements are displaced or spaced apart fromone another as required to achieve the phase relationship describedabove in connection with FIG. 13. In addition, the actual physicalelectrode pattern and placement of the elements on band 172 can bedetermined by finding intersection loci between band 172 as wrappedaround vessel 170 and equally-spaced planes. The spacing between theseplanes is defined by the equation noted above for the projected spacing.

Conductors 180 that connect transducer elements of the same phase differby 90°. There are two ways to achieve the 90° phase variation betweenthe ultrasonic waves produced by successive electrodes in the conformalarray. In the first approach, a uniformly polarized piezoelectricplastic substrate is used and every fourth element is connectedtogether, producing four groups of elements or electrodes that produceultrasonic waves having phasal relationships of 0°, 90°, 180°, and 270°,respectively. Alternatively, a zone polarized piezoelectric plasticsubstrate could be used and every other element can be connectedtogether (as shown in FIG. 15). Each of these two groups is thenconnected to provide an in phase and a quadrature phase transceivingsystem, so that ultrasonic waves are produced by the elements having arelative phase relationship of 0° and 90°. In the first approach, amulti-layer interconnect pattern is required to connect to all tracesfor each of the transducer elements in the four groups. In addition, amore complex four-phase electronic driving system that includes a phaseshifter is required. Specifically, the signal applied to each of thefour groups must differ by 90° between successive elements to achievethe 0°, 90°, 180°, and 270° driving signals. The phase shifter, e.g.,included in the modulator that drives the transducer, provides the phaseshifted excitation signals applied to each successive element of thetransducer.

In the second approach, which is preferred because it simplifies theelectronic package required and because it facilitates use of a simpler,double-sided electrode pattern, the piezoelectric plastic material mustbe locally polarized in a specific direction, depending upon the desiredphase of the electrode at that location. A polarity reversal provides a180° phase shift, eliminating the need for 180° and 270° electronicsignals. Thus, the zones of the substrate designated as 0 and 90° wouldbe connected to the signal source with the poles of the elements in onedirection, while zones for elements designated to provide a relativephase shift of 180° and 270° would be connected with the poles of theelements in the opposite direction. Elements producing ultrasonic waveswith a relative phase relationship of 0° and 180° would comprise onegroup, and elements producing ultrasonic waves with a relative phaserelationship of 90° and 270° would comprise a second group. Connectingthe poles of the different groups in local regions in oppositedirections is achieved by applying electric fields of opposite polarityin those areas during manufacture of the conformal array transducer. Thefinal element wiring pattern required to actually energize the conformalarray transducer when it is employed for monitoring flow and/or velocityof fluid through the vessel would preclude applying electric fields inopposite polarity. Accordingly, the required poling relationship wouldhave to be performed using either temporary electrodes or by providingtemporary breaks in the actual electrode pattern employed in the finalconformal array transducer.

In the preferred embodiment, to achieve a desired frequency ofoperation, it is contemplated that the electrode mass would be increasedto a point well beyond that required for making electrical connections.This added mass would act together with the piezoelectric plasticmaterial to form a physically resonant system at a desired frequency. Inthis manner, a relatively thinner and more flexible piezoelectricplastic material can be used for the substrate comprising band 172. Useof mass loading in this manner is well known to those of ordinary skillin the art of transducer design, at least in connection with producinglarge, single element, piston transducers.

The conformal array transducers can be formed on band 172, butalternatively, can be included within the structure of a syntheticgraft. FIG. 16 illustrates a portion of a cross-sectional view of theconformal array transducer fabricated on band 172. The entire transducerassembly is covered with an outer coating 190 made from a biocompatiblematerial that serves as a barrier to protect the conformal arraytransducer from bodily fluids. Below the outer coating is an RF shield192, comprising electrically conductive flexible material or a thin foilthat provides RF shielding to minimize EMI radiated from the conformalarray transducer assembly. An acoustic backing 194 comprising aconventional, or a syntactic foam, i.e., a polymer loaded with hollowmicrospheres, such as is well known to those of ordinary skill in theart, serves both for acoustic isolation and dampening and to minimizecapacitive loading. The acoustic backing has a relatively low dielectricconstant, thereby minimizing capacitive loading between the electrodesand surrounding tissue. Acoustic backing 194 thus insulates thetransducer elements from the surrounding fluid and tissue in acapacitive sense, and also in an acoustic sense. The next layer radiallycloser to the longitudinal center of the vessel comprises a rearelectrode 196. A front electrode 200 is spaced apart from the rearelectrode by a piezoelectric plastic layer 198. As noted above, in thepreferred embodiment illustrated in FIGS. 13-15, piezoelectric plasticlayer 198 comprises band 172. Piezoelectric layer 198 (or band 172) hasa relatively low dielectric constant, e.g., from about six to eight)compared to tissue (approximately 80).

Rear electrode 196 and front electrode 200 preferably comprisemulti-layer structures (although separate layers are not shown). Forexample, the electrodes will include a metallic layer that bonds well tothe piezoelectric plastic material, for example, titanium, followed by ahighly conductive layer, for example, copper, followed by an oxidationresistant layer, for example, gold. Such multi-layer systems are wellknown in the field of electronic interconnects and are ideally suitedfor use as electrodes in the conformal array transducer. Preferably,front electrode 200 is the "common electrode" for the transducerelements and serves as an RF shield. A front coating 202 serves as anacoustic coupling between the conformal array transducer and the vesselabout which it is applied. In addition, the front coating layer servesas a biocompatible layer, providing a barrier to fluid ingress into theconformal array transducer. The transducer assembly comprising each ofthe layers disclosed above is wrapped around and in contact with avessel wall 203 as shown in FIG. 16.

Referring now to FIG. 17, an embodiment of the conformal transducerarray fabricated as an integral component of a wall of a synthetic graftis illustrated (only a portion of a cross section showing the pluralityof layers comprising the device is illustrated). A synthetic graftmaterial 204 provides the primary structure for the synthetic graft andis adapted to be installed in a patient's vascular system. Typically,the graft material will comprise either a foamed fluoropolymer such asthat sold by Gortex Corporation, or a fabric such as DACRON. The graftmaterial is characterized by a moderate attenuation of ultrasonicsignals and a structure that is somewhat porous to bodily fluids. Belowgraft material 204 is disposed outer coating 190 comprising abiocompatible material that protects the transducer elements, and othercomponents of the transducer system from bodily fluids that may permeatethe graft material. Below outer coating 190 is disposed RF shield 192,to minimize transmission of EMI outside the patient's body. Acousticbacking 194 is disposed between RF shield 192 and rear electrode 196,and as described above, is a relatively lossy material. Piezoelectricmaterial 198 couples rear electrode 196 and front electrode 200 andcomprises one of the flexible piezoelectric plastics noted above. Frontcoating 202 is applied to the inner surface of the graft and transducerassembly and is selected for its biocompatibility, to withstand exposureto the bodily fluids flowing through the graft.

In both the conformal array transducer assembly provided in band 172 (asshown in FIGS. 13-15) and the transducer assembly included within thestructure of the synthetic graft wall, as illustrated in FIG. 17, it iscontemplated that adhesive layers (not shown) may be used between thevarious layers. However, certain layers such as front and rearelectrodes 200 and 196 will likely need not be adhesively coupled to thepiezoelectric material if photolithographically formed on the material.Other layers may not require an adhesive to couple to adjacent layers,e.g., if formed of a thermoset material that self bonds to an adjacentlayer when set.

As noted above, one of the advantages of the conformal array transduceris its relatively low profile. In some cases, a synthetic graft mayaccommodate a relatively thicker profile transducer assembly within itswall. An embodiment of a tilted element transducer 210 is illustrated inFIG. 18. Each element comprising tilted element transducer 210 includesrear electrode 196 and front electrode 200 disposed on opposite sides ofpiezoelectric material 198. Conventional prior art transducers forproducing an ultrasonic waves use a single such element that has asubstantially greater width that is too great for inclusion in the wallof a graft. In contrast, tilted element transducer 210 includes aplurality of elements like those shown in FIG. 18 that minimize theradial height (or thickness) of the transducer.

The tilted element transducer is built into the wall of the syntheticgraft, generally as shown in FIG. 18 and includes coating 190, whichagain serves the function of providing a biocompatible layer to protectthe interior portion of the graft and the transducer componentscontained therein from exposure to bodily fluids outside the graft.Inside outer coating 190 is synthetic graft material 204, whichcomprises the overall structure of the graft. RF shield 192 extends overthe portion of the graft in which tilted element transducer array 210 isdisposed within the protection provided by coating 190. Below RF shield192 is disposed acoustic backing 194.

An acoustic filler material 212 is disposed between front electrode 198and front coating 202, on the interior surface of the synthetic graft,and is used to fill in the cavities in front of the transducer elements.The acoustic filler material is characterized by a relatively lowultrasonic attenuation, so that it readily conveys the ultrasonic wavesproduced by the elements into the lumen of the graft. In order tominimize reverberations of the ultrasonic waves in this acoustic fillermaterial, its acoustic impedance, which is equal to sound velocity timesdensity, is approximately equal to that of the fluid in the vessel. Thevelocity of sound in the acoustic filler material should also be closeto that of the fluid flowing through the graft so that the sound beam isnot significantly deflected by the acoustic filler material.Alternatively, an acoustic filler material having a relatively low soundvelocity compared to the fluid may be used. In this case, the acousticfiller material acts as an acoustic lens that deflects the sound beingproduced by the tilted transducer elements, for example, materials suchas silicones or fluorosilicones, typically having sound velocities about1000 meters per second (compared to a sound velocity of approximately1540 meters per second for blood), may be used. Low velocity lenses aregenerally well known in the art of ultrasonic transducers. The benefitof using a low velocity acoustic filler material is that the tiltedtransducer elements can be tilted about 30% less than would be requiredotherwise. As a result, the overall height of the tilted elementtransducer portion of the synthetic graft can be made about 30% thinnerthan would be possible without the low velocity acoustic fillermaterial. In combination, the plurality of tilted elements produce anultrasonic wave 214 that propagates at an angle relative to thelongitudinal axis of the synthetic graft, which is represented by acenter line 216 in FIG. 18.

In FIGS. 19A and 19B, an artificial graft 220 is illustrated in whichpressure transducers 226 are incorporated within the wall of the graftfor monitoring the pressure of fluid passing through the graft. Layer190 of the biocompatible material is disposed between graft material 204and transducers 226 and is employed to protect the transducers, andother components of the transducer system from bodily fluids that maypermeate the graft material. As shown in FIG. 19B, two pressuretransducers 226 are employed, one being used for monitoring the proximalfluid pressure and the other for monitoring the distal fluid pressure.To accommodate measurements of proximal and distal fluid pressure,pressure transducers 226 are disposed adjacent the entrance and exitends of artificial graft 220, respectively.

Pressure transducer 226 may comprise one of several different types ofdevices for sensing pressure. Such devices include an integrated circuitpressure sensor, a strain-type pressure sensor, such as a resistivestrain gauge that responds to fluid pressure, etc. Various types ofpressure sensing devices appropriate for incorporation in the wall of agraft are readily available from a number of different commercialsources. Pressure transducers 226 are coupled through leads 232 to animplantable electronic circuit 230, such as that illustrated in FIG. 6,as discussed above. A line 234 connects the circuit to a remotelydisposed RF coupling coil (not shown in FIG. 19B) like one of thosediscussed above in connection with FIGS. 7 and 8, or to one disposedwithin the wall of the graft, as also discussed above. The interiorsurface of synthetic graft 220 includes an internal coating 224 thatconveys pressure readily from the fluid flowing through an interior 222of the artificial graft to pressure transducers 226. Inner coating 224is biocompatible and comprises an elastomeric material.

In FIGS. 20A and 20B, an alternative approach for monitoring thevelocity of a fluid through an interior 250 of a graft 240 isillustrated. In this embodiment, a pair of ultrasonic transducers 242aand 242b are mounted in relatively close proximity within the wall 244of synthetic graft 240. Alternatively, the ultrasonic transducers may bedisposed externally in contact with the outer surface of a natural graft(not shown). Ultrasonic transducers 242a and 242b each produce a pulseand receive the echo back from fluid flowing through interior 250 of thegraft, the echoes being scattered from the fluid flowing therein. Inthis embodiment, the signals 246 received from transducer 242a inresponse to the echo is correlated with the similar signal 248 fromultrasonic transducer 242b, resulting in a time delay estimate. Thevelocity of the fluid is then computed by dividing a distance betweenthe center of transducer 242a and the center of transducer 242b by thetime delay that was determined from the correlation analysis.

Unlike a Doppler system, the echoes in a correlation type transducersystem like that shown in FIGS. 20A and 20B are not frequency shifted.Instead, the velocity signal is extracted by correlating the echoamplitude versus time signals for a pair of range bins. The velocityversus time is independently determined for each range bin, resulting ina time dependent velocity profile across the diameter of the vessel orgraft.

Although the present invention has been described in connection with thepreferred form of practicing it, those of ordinary skill in the art willunderstand that many modifications can be made thereto within the scopeof the claims that follow. Accordingly, it is not intended that thescope of the invention in any way be limited by the above description,but instead be determined entirely by reference to the claims thatfollow.

The invention in which an exclusive right is claimed is defined by thefollowing:
 1. A system for monitoring a parameter indicative of acondition of a vessel relating to its ability to convey a fluid, saidsystem comprising:(a) a carrier band that is adapted to couple about thevessel so that the carrier band is in close proximity to at least oneside of the vessel; (b) a first transducer having a plurality ofconformal elements disposed in a spaced-apart array on the carrier band,said plurality of conformal elements being sufficiently flexible andshaped so that they are adapted to curve about the vessel, conforming toits shape, said first transducer being adapted to couple to a telemetryradio frequency signal and producing ultrasonic waves when excited bythe telemetry radio frequency signal that are emitted from the pluralityof conformal elements and are directed into an interior of the vessel;and (c) a receiver disposed to receive the ultrasonic waves after theyhave propagated at least partially through the vessel, said receiverproducing a signal indicative of an effect on the ultrasonic waves dueto the fluid in the vessel and thus, indicative of a parameter relatingto a flow of the fluid through the vessel.
 2. The system of claim 1,wherein ultrasonic waves are emitted as a pulse, the first transduceralternately comprising an emitter of the pulse of the ultrasonic waves,and then comprising the receiver, said pulse of ultrasonic waves beingreflected from the fluid in the vessel back toward the first transducer.3. The system of claim 1, wherein the vessel comprises a graft adaptedto be disposed within a patient's body, said system being used tomonitor the flow of the fluid through the graft to determine whether theflow of the fluid through the graft is being blocked.
 4. The system ofclaim 3, further comprising a coil adapted to receive power from anexternal source and to provide an electrical current that is used forenergizing components of the system.
 5. The system of claim 4, whereinthe coil is disposed within a wall of the graft.
 6. The system of claim4, wherein the coil is disposed on the carrier band.
 7. The system ofclaim 4, further comprising a power supply, and a multiplexer, bothbeing adapted to be internally disposed within the patient's body, saidmultiplexer alternately coupling the coil to the receiver and then tothe power supply, so that power from the external source is supplied tothe power supply when the coil is coupled to the power supplied by themultiplexer, and the signal produced by the receiver is supplied to thecoil when the coil is coupled to the receiver.
 8. The system of claim 1,further comprising a second transducer having a plurality of conformalelements disposed in a spaced-apart array on the carrier band, saidplurality of conformal elements being sufficiently flexible and shapedso that they are adapted to curve about the vessel, conforming to itsshape, said first transducer being and are directed into an interior ofthe vessel.
 9. The system of claim 8, wherein the carrier band comprisesa substrate of a piezoelectric material on which a plurality ofelectrodes are formed to define the conformal elements of the first andthe second transducers.
 10. The system of claim 9, wherein thepiezoelectric material is uniformly polarized, every fourth electrode ofthe plurality of electrodes being coupled together.
 11. The system ofclaim 9, wherein the piezoelectric material is zone polarized, everyother electrode comprising the plurality of electrodes being coupledtogether.
 12. The system of claim 9, wherein the carrier band furthercomprises an acoustic backing layer encompassing the plurality ofelectrodes that are formed on the piezoelectric material.
 13. The systemof claim 9, wherein the carrier band further comprises a radio frequencyshield encompassing the plurality of electrodes formed on thepiezoelectric material.
 14. The system of claim 1, wherein theultrasonic waves emitted by successive elements of the first transducerare 90° out of phase, so that the ultrasonic waves emitted in onedirection are canceled due to a destructive interference.
 15. The systemof claim 8, wherein said second transducer is adapted to couple to theradio frequency signal and produces ultrasonic waves when excited by theradio frequency signal, said first transducer and said second transduceralternately interchangeably functioning as the receiver and as anemitter of the ultrasonic waves during successive time intervals. 16.The system of claim 8, wherein the first transducer and the secondtransducer are disposed on opposite sides of the carrier band when it isfitted about the vessel, said second transducer comprising the receiverof the ultrasonic waves emitted by the first transducer, said parameterbeing determined as a function of a transit time for the ultrasonicwaves to pass through the vessel.
 17. The system of claim 8, wherein theconformal elements of the first transducer and of the second transducerare shaped like segments of a sine wave having a period substantiallyequal to a circumference of the vessel.
 18. The system of claim 17,wherein an amplitude of the sine wave having segments relating to theshape of the conformal elements substantially determines a beam angle ofthe ultrasonic waves.